1. Field of the Invention
The present invention relates to a perpendicular recording magnetic disk apparatus.
2. Description of the Related Art
A perpendicular recording magnetic disk apparatus presently being developed uses a method in which writing to a so-called perpendicular two-layered medium having a soft underlayer (SUL) and a perpendicular recording layer is performed by using a single-pole head. In this perpendicular recording magnetic disk apparatus, data is written in the perpendicular recording layer by employing the magnetic coupling between the single-pole head and the soft underlayer, i.e., the magnetic flux which flows from a main pole to a return yoke through the soft underlayer. From the standpoint of signal quality, it is important that the magnetization in the soft underlayer be stable.
One requirement for the perpendicular recording magnetic disk apparatus is little medium noise. More specifically, it is desirable that no spike noise be produced, colored wideband noise be little as a whole, and the frequency characteristics be small. However, since the coercivity Hc of the soft underlayer is small, it often forms magnetic walls under the influence of a magnetic field. When the head passes over a magnetic wall, spike noise is produced in the read signal. Therefore, it is desired that magnetic walls rarely be formed in the soft underlayer. To achieve this object, a structure is used which fixes the magnetization in the soft underlayer in a predetermined orientation by forming below the soft underlayer a pinning layer made of, e.g., a hard magnetic material.
Conventionally, a medium in which magnetic walls are rarely formed even when a magnetic field is applied to the soft underlayer has been proposed (Jpn. Pat. Appln. KOKAI Publication No. 2003-109201). In this reference, on the basis of an M-H curve in the in-plane direction of the soft underlayer, the intersection of a straight line connecting a point (Hs, Ms) at which magnetization saturates in the first quadrant and a positive intersection (0, Mr) of the M-H curve and the ordinate, and a tangent at a negative intersection (−Hc, 0) of the M-H curve and the abscissa is obtained, and an absolute value Hn of a magnetic field at this intersection is used as a parameter. In this perpendicular recording medium, the maximum magnetic field applied longitudinally to the magnetic recording medium is smaller than the magnetic field Hn. Therefore, the magnetization of the soft underlayer varies with retaining the value of the saturation magnetization, and the M-H curve does not form a minor loop. This makes it possible to keep the magnetizations oriented in the same direction by the exchange coupling interaction between the soft underlayer and pinning layer.
If the exchange coupling between the pinning layer and the soft underlayer is too strong, the magnetization in the pinning layer may reverse together with the magnetization in the soft underlayer when a magnetic field is applied. Accordingly, a proposal which suppresses magnetization deviation in the soft underlayer by appropriately weakening the exchange coupling force between the pinning layer and the soft underlayer is also known (Jpn. Pat. Appln. KOKAI Publication No. 2003-162807). More specifically, in a magnetic recording medium in which an in-plane hard underlayer, an intermediate layer, an in-plane soft underlayer and a magnetic recording layer are stacked on a substrate, the intermediate layer is formed of a material selected from the group consisting of a magnetic layer having saturation magnetization smaller than that of the in-plane hard underlayer, a nonmagnetic layer having a thickness of 0.5 nm or less and an oxide layer of the in-plane hard underlayer.
A cause that makes the magnetization of the soft underlayer in the perpendicular two-layered medium deviate from a predetermined direction includes the magnetic field applied by the write head to the pinning layer. To obtain a high write performance, a head magnetic field in the center of the recording layer must be strengthened. However, when this head magnetic field becomes strong, the magnetic field applied to the pinning layer also becomes strong. If the magnetic field from the write head is too strong, therefore, magnetization deviation may occur in the soft underlayer.
When the manufacturability of the medium is taken into account, a thinner soft underlayer is presumably more preferable because the uniformity of the soft underlayer can be increased and dust can be reduced. Although the thickness of a conventional soft underlayer is 200 to 250 nm, the aimed thickness for the future is preferably 150 nm or less, and more preferably, 100 nm or less. When the soft underlayer is thus made thinner, the field strength applied by the write head to the pinning layer increases further, and the magnetization in the soft underlayer may easily deviate. Unfortunately, a design criterion of a perpendicular recording disk apparatus for preventing magnetization deviation in the soft underlayer has yet to be established.